Power supply apparatuses are essential for many electronic appliances such as personal computers, industrial computers, servers, communication products or network products. Usually, the user may simply plug a power supply into an AC wall outlet commonly found in most homes or offices so as to receive an AC voltage. The power supply can convert the AC voltage into a regulated DC output voltage for powering the electronic device and/or charging a battery built-in the electronic device.
Generally, power supply apparatuses are classified into two types, i.e. a linear power supply and a switching power supply (SPS). A linear power supply principally comprises a transformer, a diode rectifier and a capacitor filter. The linear power supply is advantageous due to its simplified circuitry and low fabricating cost. Since the linear power supply has bulky volume, the linear power supply is not applicable to a slim-type electronic device. In addition, the converting efficiency of the linear power supply is too low to comply with the power-saving requirements. In comparison with the linear power supply, the switching power supply has reduced volume but increased converting efficiency. That is, the switching power supply is applicable to the slim-type electronic device and may meet with the power-saving requirements.
For maintaining normal operation of the electronic device, the switching power supply needs to provide sufficient power to the electronic device. For example, if the maximum power consumed by the electronic device is 400 watt, the switching power supply is usually designed to provide power of 400 watt or more. In addition, since the switching power supply generates the DC voltage for powering the electronic device by selectively conducting or shutting off many switch elements, many switch elements are necessary to provide a greater magnitude of power. As known, the switch elements are still selectively conducted or shut off even when the switching power supply is operated in a standby status or a power-saving status. The process of selectively conducting or shutting off the switch elements results in additional switching loss. In addition, the overall efficiency of the switching power supply is impaired.
Moreover, since the AC current inputted into the switching power supply is centrally distributed, the harmonic wave generated by the switching power supply may interfere with other electronic devices, reduce the power factor and increase the apparent power. Under this circumstance, the switching power supply needs more power and the power loss is high.
Therefore, there is a need of providing a single-stage switching power supply to obviate the drawbacks encountered from the prior art.